Developments in Ceramic Materials Research

232
R. Ramesh, H. Kara, Ron Stevens and C. R. Bowen
Piezoelectric charge coefficients, permittivity and impedance data are collected using the
method described in Section.2.1.
The measured piezoelectric and dielectric properties of the dense PZT ceramic and
piezocomposites are given in Table 2. This table shows that the reduction in d31 is
significantly greater than that for d33. Consequently, gH and figure of merit (dH.gH) are
improved for the piezocomposites compared with dense PZT. However, the dielectric
constant and hence, the capacitance of the piezocomposites is much lower than that of the
dense piezoceramic material.
Table 2. Properties of dense piezoceramic, 22% PZT-Air composite and 11% PZT-
Polymer composite. The parameters d33, d31, , device capacitance and receiving
sensitivity have been measured and the parameters dH, gH are dH.gH are calculated
Material d 33
pCN -1
d 31
pCN -1
T
ε 33 x10 -9
Fm -1
ε
dH
pCN -1
T
33
g H x10 -3
VmN -1
d H.g H
x10 -15
Pa -1
MdB re.
1VμPa -1
Device
Cap.pF
PZT-5H 513 220 25.3 72.8 2.87 209 -205 5050
22% PZT-Air 210 29 2.8 152 54.2 8238 -198 800
11% PZT-
Polymer
35.7 7 0.67 21.7 28.3 614 -215 210
5.1.2. Hydrophone Assemby
The porous piezocomposites show a low permittivity, which results in a low measured
device capacitance values compared to the dense PZT hydrophone (Table 2). Since the
capacitance of the cable used (RS Products) is 42 pF m -1 and a 1.5m cable used for each
hydrophone, the capacitance of the devices is increased in order to reduce the cable loading
effect which can lead to a decrease in the hydrophone sensitivity. To achieve this, two discs
of piezocomposite are attached back-to-back using an electrically parallel connection. The
configuration is shown in Figure 14. The pieces are attached to a 0.15 mm thick copper plate
electrode with conductive epoxy (RS Products, UK, Product 186-3616) in opposite poling
directions. The remaining two faces are then attached with copper electrodes and soldered to
a cable. The middle electrode is used as the positive lead and the two outer electrodes are
connected to the cable shield. Finally, the whole assembly is molded in 1mm thick
polyurethane for water isolation (Figure 14). The dense PZT ceramic hydrophone is also
assembled in the same way with the same thickness for a direct comparison. In order to
ensure the structural integrity of the device assembly, an impedance spectrum is measured, in
air, at every stage of the construction using Solartron Impedance Analyser (Model 1260,
USA). The impedance spectra so recorded do not show any spurious peaks, indicating proper
structural integrity of the hydrophones.
For any underwater application, hydrophones are normally used in the form of an array.
Hydrophone arrays covering large surface area are desirable for certain applications because,
it enhances the signal-to-noise ratio of the receiver system. Further, acoustic imaging systems
require arrays of hydrophones to collect information arriving from different angles with
respect to the position of the array itself. Characteristics of hydrophone arrays are found to be